The mobility afforded by wireless communication networks has resulted in increased usage of such networks. Private wireless networks and cellular networks are two common types of wireless communication networks. Private wireless networks, also known as wireless local loop (WLL) networks, are commonly operated by public safety agencies, taxi services and delivery services. Private wireless networks typically operate over a limited range of frequencies and within limited geographic areas. In contrast, cellular networks typically operate over a larger number of frequencies and provide coverage over larger geographic areas.
Although conventional cellular networks may provide sufficient reliability for the average user, there are a number of deficiencies which prevent widespread adoption by public safety agencies. For wireline communication public safety agencies can be provided with dedicated circuits and switches such that even when the Public Switched Telephone Network (PSTN) is overloaded with non-emergency traffic, communications between, and within, public safety agencies can still be completed. To provide reliability to wireless communications, public safety agencies typically employ private wireless networks which operate over frequencies reserved for public safety agencies.
Although these private radio networks reduce the likelihood that calls by public safety agencies are blocked from accessing the radio network, they are expensive to implement and maintain. For example, these networks typically require the use of specialized mobile stations which are more expensive than typical mobile stations, due to the relatively low demand for the specialized mobile stations compared to that of mass-produced mobile stations. As used herein, the term mobile station (MS) is intended to encompass any type of wireless communication device including wireless telephones, wireless Personal Digital Assistants (PDA), wireless pagers, portable computers with wireless modems and the like.
Compared to cellular networks, private wireless networks are more likely to have dead spots where a radio signal cannot be received by the public safety agency worker's mobile station. These dead spots can be extremely hazardous to the public safety agency workers, e.g., a police officer requesting backup, and to the citizenry in general, e.g., a public safety agency worker requesting an ambulance or fire trucks. Accordingly, public safety agencies desire the coverage area provided by conventional cellular networks and the reliability provided by private radio networks.
FIG. 1 illustrates a conventional communication network. In the conventional communication network of FIG. 1, a wireless carrier's cell sites 110a and 110b, are respectively coupled to base transceiver stations (BTSs) 120a and 120b. The wireless carrier relies upon connections through a public switched telephone network (PSTN) to provide a backhaul connection between the BTSs and the wireless carrier's core network 150. Specifically, each of the BTSs 120a and 120b are respectively coupled to a central office 130 of the PSTN via communication links 142 and 144. Typically, communication links 142 and 144 are T1 communication links. T1 communication links are digital communication links that have a large bandwidth, i.e., 1.544 Mbps. T1 communication links are leased from a PSTN operator by a wireless carrier, and result in significant monthly recurring costs.
A central office 130 aggregates a number of T1 links and forwards the information to a wireless carrier core network 150 via a high bandwidth communication link 148, such as a channelized digital signal level 3 (DS-3) communication link. A channelized DS-3 communication link carries approximately 44.736 Mbit/sec of information. The wireless carrier core network 150 receives the channelized DS-3 communication link by an add-drop multiplexer 160. The add-drop multiplexer 160 places the information on a network 165, which provides the information to an add-drop multiplexer 170 within a mobile switching office (MSO) 175. The network 165 can be any type of network, e.g., a synchronous optical network (SONET).
If the T1 communication link 142 or 144 between the BTSs 120a and 120b and the central office 130 fails, then all communications for the particular cell site fail because the communications cannot be forwarded to the wireless carrier core network 150. One technique for addressing the failure of the T1 connection is to provide a redundant T1 connection 146. Accordingly, if the T1 connection 142 fails, the BTS 120a can still communicate with the central office 130 using T1 146. However, because the T1 connections are leased, and can be quite costly, when the primary communication link, e.g., T1 142, is operating properly the expense of the secondary communication link, e.g., T1 146, is largely wasted. T1 links between cell sites and central offices do not fail regularly, and accordingly, leasing a redundant T1 connection is a costly way to address a problem which rarely occurs.
Although failure of T1 links rarely occurs, to certain wireless users, such as public safety agencies, high availability is required for any communication network. Accordingly, it would be desirable to provide techniques for fault tolerant wireless communications with a minimum of expense.